A conventional power transmission system employing a motor (110) and an engine (10) is shown in FIGS. 1 and 2 and comprises a first pulley means (11) connected to the motor (110) and a second pulley means (14) connected to the engine (14). A gear box (12) has a shaft (121) whose two ends are respectively connected to two third pulleys (15) which are respectively connected to the first pulley means (11) and the second pulley means (14) by two belts (111, 141). Each of the two third pulleys (15) has a uni-directional device (13) connected thereto. It is desired to employ the output power of the engine (10) or the output power of the motor (110) according to the situation such as a high torque required situation or a high speed required situation. Each of the pulley means (11, 14, 15) has a variable width for movably receiving the V-shaped belts (111, 141) to be engaged so as to generate different speed and torque. There have some inherent shortcomings which are:
1. The system involves the first pulley means (11) and the second pulley means (14) each of which comprises a plurality of balls which move within the casing of the respectively pulley means according to the speed of the pulley means they are involved, two general pulleys (15) and two belts (111, 141). The system therefore occupies a large space. PA1 2. The system cannot provide enough torque in time when riding the vehicle or motorbike on a rugged road or snow-covered road. That is because the balls in the first pulley means (11) and the second pulley means (14) cannot move outward when the rotational speed is low so that the system cannot provide enough torque under a lower r.p.m. of the motor (110) or the engine (10). PA1 3. The speed reduction ratio is so small that the conventional system cannot meet the needs of practical use.
The present invention intends to provide an improved power transmission system which timely shifts the output power of the motor or the engine and generates a large speed reduction ratio. The present invention has arisen to resolve the shortcomings of the conventional power transmission system.